Semiconductor technology develops rapidly. 10-nanometer, or less, fabrication process is ready to come to the market. A semiconductor product is fabricated by tens, or hundreds of steps. Errors will accumulate. Therefore, precise alignment is an important issue.
Mechanical alignment and optical alignment are two popular ways to align two wafers. Mechanical alignment takes the notch or flat of a wafer as a basis for alignment, and uses special pins to align the wafer. Optical alignment, e.g., direct alignment, allows visible or infrared light to pass wafers, and uses optical instruments to align the wafers by reference to positioning marks disposed thereon.
However, these alignment mechanisms are not qualified to address the 10-nanometer fabrication process.